Devices (such as car stereos) that can receive audio through the 30-pin cable did so using the old IPod playback protocol. Apple has been deprecating and removing support for that protocol for years. Chances are, even if you find the right combination of cables and/or adapters to get a modern iPhone connected to it, it won’t do much. It will probably only charge the iPhone, very slowly, and nothing else.
The link to a cable that someone else posted specifically states that it does not support audio or video output. Unless you choose to upgrade your car stereo, your options are limited to a headphone or Bluetooth FM radio transmitter. Years ago I tried a few of those and did not find a good one.